rmlui-iphone

Add README with build instructions and architecture notes

Phase 4: Discord-style UI, touch scroll, momentum, text input, keyboard avoidance

Fix: replace non-existent MathTypes.h with Math.h

Phase 3: gradient shader (linear, radial, conic, repeating variants)

Phase 2: stencil-based clip masks (EnableClipMask / RenderToClipMask)